1. Field of the Invention
The present invention relates to an electronic endoscope system. Particularly, the present invention relates to a mask operation for image signals that are obtained by an image-pickup device.
2. Description of the Related Art
In recent years, electronic endoscopes have been widely used as effective devices for instant observation of an internal space, such as the inner wall of digestive organs and the like. An electronic endoscope comprises a flexible conduit (or insertion portion) that is inserted into an organ. At the distal end of the flexible conduit, an image-pickup device, which is associated with an objective lens, is mounted. The image-pickup device feeds image signals of the subject image to an exclusive image-signal processing unit or apparatus, so that the image processor reproduces a color image of the subject on a monitor screen in accordance with the image signals.
The depth of field of the objective lens, which is fixed at the distal end of the flexible conduit, is set relatively deep, so that an image of an internal organ can be captured in-focus over a wide range, even though the focus of the optical system is fixed. Namely, a relatively short focal length is provided for the objective lens. However, when the focal length of the objective lens is short so as to provide a deep depth of field without narrowing the stop, an image on the imaging surface (or image plane) of the image-pickup device becomes strongly blurred as the radial distance from the optical axis increases due to the aberration of the objective lens, such as an astigmatism. Therefore, the outer peripheral region where the strong blur appears is masked, and only the in-focused area, the area on which a clear image is produce, is displayed on a monitor screen.
An example of masking is an optical mask that is arranged at the above outer peripheral area of the imaging surface of the image-pickup device. Namely, the optical mask optically shields the outer peripheral area from the incident light, and image signals output from the masked outer peripheral area are made substantially equal to the black level. Another example of masking is an electronic mask (or mask operation). The electronic mask is achieved by replacing image signals corresponding to the above outer peripheral area of the imaging surface with black level signals in an image-processing unit. In a conventional masking method, an area to be masked (in the following referred to as a masking area) is located at the peripheral area of the in-focus area, and the masking area is previously defined in the design stage by regarding the size of the in-focus area and its position relative to the imaging surface.
Since the flexible conduit of an electronic endoscope is inserted into a narrow space like an organ, the maximum diameter for the conduit is a couple of centimeters at most, so that the size of the image-pickup device and the objective lens is a minimum. These facts give rise to the requirement for precise attachment of the image-pickup device and the objective lens, and for accurately applying the optical mask to the imaging surface of the image-pickup device. Therefore, as a practical matter, the positions of the in-focus area, the imaging area (effective image-sensing area or image-output area), and the masking area are not appropriately aligned as a result of a small relative displacements between the optical axis of the objective lens, the center of the imaging area, and the center of the optical mask. Further, these errors in the alignment are peculiar to each particular electronic endoscope.
Namely, when an electronic endoscope, which is connected to the image-signal processing unit, is changed to a different one, the alignment of the objective lens, the imaging area, and the optical mask is also changed, so that an undesirable part, such as the out-focus area may appear on the monitor.